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Amenorrhea from Low Body Weight: Hormonal Mechanisms & Recovery Pathways

Pathophysiology: The "Energy Availability" Model

Low energy availability (LEA) = dietary energy intake − exercise energy expenditure < 30 kcal/kg FFM/day → hypothalamus suppresses GnRH pulse generator → hypogonadotropic hypogonadism.

Hormonal Cascade

  1. Low leptin (from low adipose) → signals energy deficit to hypothalamus.
  2. GnRH pulse frequency ↓ → LH/FSH pulsatility disrupted.
  3. Ovarian follicular arrest → low estradiol/progesterone.
  4. Endometrium remains thin → no withdrawal bleed = amenorrhea.
  5. Cortisol ↑ (stress response) → further suppresses GnRH.

Diagnostic Criteria (Functional Hypothalamic Amenorrhea - FHA)

  • Amenorrhea ≥3 months (or oligomenorrhea >45 days).
  • Low/normal FSH, LH, estradiol.
  • Exclusion of organic causes (pregnancy, thyroid, prolactin, PCOS, POI).
  • Associated: low BMI (<18.5), high exercise, stress, disordered eating.

Health Consequences

  • Bone: ↓ BMD → osteopenia/osteoporosis, stress fractures (irreversible if peak bone mass not achieved).
  • Cardiovascular: Endothelial dysfunction, lipid abnormalities.
  • Metabolic: Insulin sensitivity paradox (high but low energy).
  • Psychological: Anxiety, depression, obsessive traits.
  • Fertility: Anovulation; reversible with weight restoration.

Recovery Roadmap

StepAction
1. Medical AssessmentRule out organic causes; DEXA (Z-score), labs (TSH, prolactin, FSH, LH, estradiol, CBC, metabolic).
2. Nutritional RehabilitationRegistered dietitian; target ≥30 kcal/kg FFM/day; weight gain 0.5–1 kg/wk until BMI ≥18.5–19.
3. Exercise ModificationReduce intensity/volume; eliminate compulsive exercise; add rest days.
4. Psychological SupportCBT-E (enhanced for eating disorders), ACT, family therapy if adolescent.
5. Hormonal Therapy (Adjunct)Transdermal estradiol + cyclic progesterone if BMD low & no menses after 6–12 mo of LEA correction. NOT oral OCPs (mask underlying issue, don't improve BMD).
6. MonitorMonthly weights, quarterly labs, DEXA q1–2yr; menses return = success marker.

Multidisciplinary Team Is Essential

Physician + Dietitian + Therapist + (Psychiatrist if needed). Early intervention prevents irreversible bone loss.

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